From Wikipedia, the free encyclopedia.
In biology, the cell is the fundamental structural and functional unit of all living organisms. The cell theory, first developed in the 19th century, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells and that cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
3.1 Prokaryotic cells
3.2 Eukaryotic cells
- A membrane, which envelopes the cell, separates its interior from the surroundings, strictly controls what moves in and out and maintains the electric potential of the cell,
- A salty cytoplasm (the substance which makes up most of the cell volume)
- DNA, the hereditary material of genes, which guide the operations of the cell.
- RNA, through which DNA instructions are expressed.
- Enzymes and other protein machinery.
- A variety of biomolecules.
Organisms vary from single cells (called single-celled organisms) that function and survive more or less independently, through colonial forms withcells living together, to multicellular forms in which cells are specialized and do not generally survive once separated. There are 220 types of cells and tissues that make up the multicellular human body.
Types of cells: prokaryotic and eukaryotic
Two basic types of cells are described: prokaryotic and eukaryotic. Prokaryotic cells are structurally simple. They are found only in single-celled and colonial organisms. In the three-domain system of Scientific classification, prokaryotic cells are placed in the domains Archaea and Eubacteria. Eukaryotic cells have organelles with their own cell membranes. Single-celled eukaryotic organisms are very diverse, but many colonial and multicellular forms also exist. (The multicellular kingdoms: Animalia, Plantae and Fungi, are all eukaryotic.)
|typical organisms||bacteria||protists, fungi, plants, animals|
|typical size||~ 1-10 µm||~ 10-100 µm (sperm cells, apart from the tail, are smaller)|
|type of nucleus||nucleoid region; no real nucleus||real nucleus with double membrane|
|DNA||circular (usually)||linear molecules (chromosomes) with histone proteins|
|RNA-/protein-synthesis||coupled in cytoplasm||RNA-synthesis inside the nucleus
protein synthesis in cytoplasm
|cytoplasmatic structure||very few structures||highly structured by intercellular membranes and a cytoskeleton|
|cell movement||flagella made of flagellin||flagella and cilia made of tubulin|
|mitochondria||none||one to several dozen (though some lack mitochondria)|
|chloroplasts||none||in algae and plants|
|organization||usually single cells||single cells, colonies, higher organisms with specialized cells|
|cell division||Binary fission (simple division)||
Mitosis (core division)
Cytokinesis (cytoplasmatic division)
- The cytoplasm of prokaryotes (the liquid which makes up most of the cell volume) is diffuse and granular due to ribosomes (protein factories) floating in the cell.
- The plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon.
- Most prokaryotes have a cell wall (some exceptions are Mycoplasma (a bacterium) and Thermoplasma (an archaeon)). It consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" from osmotic pressure against a hypotonic environment.
- A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Even without a real nucleus, the DNA is somehow condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids can carry additional functions, such as antibiotic resistance.
- Some prokaryotes have flagella which enable them to move actively instead of passively drifting.
Eukaryotic cells are highly organized and composed of structurs known as organelles that perform specific functions.
- The cytoplasm of eukaryotes does not appear as granular as that of prokaryotes, since an important part of the ribosomes are bound to the endoplasmic reticulum.
- The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
- The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are highly condensed (e.g. folded around histones). All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles can contain some DNA.
- Eukaryotes can become mobile using cilia or flagella. The flagella are more complex than those of prokaryotes.
A typical animal cell
Organelles (see diagram above)
A typical plant cell
- Central vacuole
- Rough endoplasmic reticulum
- Smooth endoplasmic reticulum
- Golgi apparatus
Human body cells
The body contains trillions of cells.
All cells share several abilities:
- Reproduction by cell division.
- Metabolism, including the taking in of raw material, using it to build cell components, or breaking it down for energy, and releasing byproducts.
- Protein biosynthesis
- The ability to respond to external and internal stimuli
- The traffic of vesicles.
Many cell functions are carried out by enzymes.
The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is derived from metabolic pathways.
Moving of proteins
A typical mammalian cell contains up to 10,000 different proteins.
The origin of cells
The origin of cells has much to do with the origin of life, and was one of the most important steps in evolution of life as we know it. The birth of the cell marked the passage from prebiotic chemistry to biological life.
Origin of the first cell
If we see life forms from the point of view of replicators, that is DNA molecules in the actual life, cells satisfy two fundamental conditions : protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and probably was the main reason for which cells evolved. The latter is fundamental for the evolution of biological complexity. If we have,let's imagine, freely-floating DNA molecules that code for enzymes that are not enclosed into cells, the enzymes that advantage a given DNA molecule (for example,by producing nucleotides) will automatically advantage also the neighbouring DNA molecules. You can see it as "parasitism by default". Therefore the evolutive pressure on DNA molecules will be much lower,since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: all molecules in a given neighbourhood are almost equally advantaged. If we have the DNA molecule enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This means other DNA molecules can't benefit of a positive mutation in a neighbouring molecule : this means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it. (Note. This is more a metaphor given for simplicity than a possible truth, since probably the earliest molecules of life, probably up to the stage of cellular life, were RNA molecules, acting both as replicators and enzymes : see RNA world hypothesis . But the core of the reasoning is the same.)
Biochemically, cell-like spheroids formed by proteinoids are observed by heating aminoacids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth.
Origin of the eukaryotic cell
The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell. There is still considerable debate on if organelles like the hydrogenosome predated the origin of mitochondria, or viceversa : see the hydrogen hypothesis for the origin of eukaryotic cells.
- 1632-1723: Antony van Leeuwenhoek teaches himself to grind lenses, builds a microscope and draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth.
- 1665 : Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.
- ...I could exceedingly plainly perceive it to be all perforated and porous, much like a Honeycomb...these pores or cells, were not very deep, but consisted of a great many little boxes... – Hooke describing his observations on a thin slice of cork.
- 1839 : Theodor Schwann and Matthias Jakob Schleiden elucidate the principal that plants and animals are made of cells, concluding that cells are a common unit of structure and development, thus founding the Cell Theory.
- The belief that life forms are able to occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822-1895).
- Rudolph Virchow states that cells always emerge from cell divisions (omnis cellula ex cellula).
- 1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University of Berlin. By 1935 he has built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
- 1953: Watson and Crick made their first announcement on the double-helix structure for DNA on February 28.
- Cell biology
- Cell division
- Cariology is the study of the cell nucleus.
- Plant cell
- Animal cell
- Fungal cell
- Prokaryotic cell
- Eukaryotic cell
- Human cell
- How to prepare an onion cell slide
- Cell types
- Wikibooks Cell Biology Textbook (http://wikibooks.org/wiki/Cell_Biology)
- Teaching about the Life and Health of Cells. (http://www.ericdigests.org/2004-1/cells.htm)
- The cell like a city (http://www.biopic.co.uk/cellcity/cell.htm).